Separation of gas mixtures containing nitrogen are important industrial processes allowing for the production of enriched oxygen and nitrogen products. These processes are practiced on a variety of scales ranging from less than 1 to greater than 100 tons per day. In the past, the primary means of air separation was by cryogenic processing. For intermediate volumes of oxygen product or production in remote areas far away from cryogenic supplies, stand-alone pressure swing adsorption units are providing significant opportunities for expanded markets.
The removal of nitrogen from air by adsorptive separation uses nitrogen selective zeolites which preferentially adsorb higher amounts of nitrogen than oxygen under a wide variety of equilibrium conditions. Oxygen enriched product made using nitrogen selective adsorbent zeolites are typically produced at elevated pressures during the air feed step, whereas the nitrogen product is formed during the desorption step and is typically at lower pressure. For a given oxygen purity, the recovery and efficiency of the process is directly related to the process scheme employed and the intrinsic adsorption properties of the zeolite.
Depending on the zeolite structure and composition, their effectiveness for adsorptive separation of air can vary over a wide range. The use of crystalline zeolitic molecular sieves in pressure swing adsorption processes for air separation are well known. In U.S. Pat. No. 3,140,931 the use of crystalline zeolitic molecular sieve material having apparent pore sizes of at least 4.6 angstroms for separating oxygen-nitrogen mixtures at subambient temperatures is disclosed. In U.S. Pat. No. 3,140,932 the strontium, barium, or nickel ion exchanged forms of X-zeolite are set forth.
In U.S. Pat. No. 3,313,091 the use of A-zeolite and X-zeolite adsorbents exchanged with Group II metal cations, such as: magnesium, calcium, strontium and barium are set forth. This patent describes ambient temperature operation at high cation exchange levels, but reports only marginal performance in its Table 1 results. It suggests that the greater cation exchange level achievable is best for performance.
U.S. Pat. Nos. 4,481,018 and 4,544,378 demonstrate an improved performance of faujasite composition containing divalent cations provided they are activated in such way that a preponderance of the polyvalent cations are in the dehydrated/dehydroxylated state. Properly activated materials showed the expected increase in nitrogen capacity and nitrogen/oxygen selectivity with increase in cation charged density from barium to strontium to calcium. In addition, these patents note the increasing calcium exchange level and greatly enhanced nitrogen/oxygen selectivity only for calcium exchange levels above 50%.
U.S. Pat. No. 4,557,736 discloses that the binary ion exchange forms of X-zeolite wherein between 5 and 404 of the available ion sites are occupied by calcium and between 60 and 95% of the sites are occupied by strontium exhibit higher nitrogen capacities at 3 to 3.5 atmospheres without adverse affects on nitrogen/oxygen selectivity or large increases in heat of adsorption compared to the single ion exchange X-zeolite with calcium or strontium.
The use of X-zeolites exchanged with monovalent cations for air separation is also known in the art. In U.S. Pat. No. 3,140,933 the use of lithium X-zeolite to separate oxygen-nitrogen mixtures at feed pressures between 0.5 and 5 atmospheres and a temperature between about 30.degree. C. and 150.degree. C. is disclosed. In U.S. Pat. No. 4,859,217 a process for selectively adsorbing nitrogen using X-zeolite having a framework silicon/aluminum molar ratio not greater than 1.5 and having at least 88% of its aluminum oxide tetrahedral units associated with lithium cations is set forth.
The use of A-zeolite for oxygen pressure swing adsorption processes which has about 70% or more of its exchangeable cations in the calcium form (commonly called 5A) is well known. This has been the most widely used adsorbent for the production of oxygen from sorptive separations on both small scales and large scale as an alternative to cryogenic production. Many improvements in the formulation and manufacture of 5A zeolite have provided effective granular, beaded or pelleted adsorbents for oxygen production. These include optimizing the activation processes Japanese 61153138-A, forming binderless granules British 1431686 and U.S. Pat. No. 3,773,690, and using nonreactive binders such as silica U.S. Pat. No. 4,950,312. Besides improvements in the 5A adsorbent formulation numerous process improvements have been developed. Recent U.S. patents describing improved oxygen processes such as U.S. Pat. No. 4,810,265 or U.S. Pat. No. 4,329,158 contain many of the relevant preferences.
In spite of all the work on A-zeolites containing calcium, there are only a few reports of the effects of magnesium containing A-zeolites on air separation properties. U.S. Pat. No. 2,882,243 discloses the utility of A-zeolites as adsorbents. However, no reference is made to the utility of magnesium containing A-zeolites for air separation and all references to any cation form are at temperatures well below ambient conditions. The use of magnesium containing A-zeolites for air separation at ambient conditions was first reported in a process patent for a vacuum swing adsorption process in U.S. Pat. No. 3,313,091. Work was reported by R. Schollner in publications and patents from East Germany between the period of 1978 to 1986. These works studied the influence of alkaline earth and alkali metal cation forms of A-zeolite for air separation, and initially carried out gas chromatograph studies showing that the high charge density of magnesium in A-zeolite greatly improves selectivity for nitrogen over oxygen. See R. Schollner, R. Broddack, M. Jusek, translation from Z. Phys Chemie, Leipzig, 262 (1981) to pages 362 to 368. This work is the first disclosure of the high nitrogen-oxygen separation factors obtainable on highly magnesium exchanged A-zeolite. Schollner recognized the importance of this observation and stated that highly exchanged magnesium A-zeolite was well suited for air separation. Schollner demonstrated air separation over magnesium, sodium A-zeolite using frontal chromatography and disclosed a process for the separation of oxygen and nitrogen from gas mixtures over magnesium A-zeolites having at least 30% magnesium and carried out in a process at temperatures less than or equal to 20.degree. C. at a pressure of 1 to 10 atmospheres. See East German patent 131,166. Schollner also demonstrated mixed magnesium, lithium A-zeolite. Finally, Schollner discloses a single step ion exchange process for exchanging calcium and magnesium on an A-zeolite useful for separation of air in an adsorptive separation. See East German 239,536.
U.S. Pat. No. 5,152,813 discloses calcium as strontium exchanged lithium X-zeolite as adsorbents for air separation.
The various reports of interest in magnesium exchanged A-zeolites fail to report a high performance magnesium A-zeolite having significant increase in performance over the standard adsorbent for nitrogen adsorption or oxygen production in pressure swing adsorption comprising calcium A-zeolite more widely known as 5A zeolite. The present invention overcomes the shortcomings of the prior art in a process providing high performance in air separation using a specially prepared magnesium A-zeolite as set forth below.